The increasing of higher level of integration within electrical integrated circuit (IC) leads to both higher data rates and larger number of IC interconnections. Today, the inherent signal speed of IC is increased to 3 GHz, and shortly it will be reached to 10 GHz and beyond. The number of pin connection is also increased, with single IC requiring close to 2000 interconnection (i.e. single processor), and shortly it will be increased to over 5000. Simultaneously achieving higher data rates and higher interconnects densities for off-chip, will be increasingly difficult as the IC technologies continue to evolve increasing signal speed of electronic devices and interconnection number. In off-chip case, high density interconnects, covering from die-level packaging to chip-to-chip (hereafter chip indicates the die with package) interconnection on the FLEX-PCB, will also be getting increasingly difficult as the IC technologies continue to evolve increasing the signal speed and interconnection number.
With increasing of the signal speed and interconnection number of the IC, low-cost high-speed interconnect technique compatible to today's manufacturing process are highly desirable to make available in consumer level. Today's FLEX-PCB is mainly made of uniform Polyimide material, and their manufacturing technology along with FLEX-PCB manufacturing are so well matured that, for long run, all system vendors like to use Polyimide based FLEX-PCB to keep the system cost low. However, Polyimide has material characteristics, which limit its usage in high speed if conventional interconnection structure is used. The reason is that conventional Polyimide has the high dielectric loss which mainly limits the bandwidth of interconnects.
FIG. 1 is the schematic showing a part of conventional FLEX-PCB. For simplicity in understanding, only a portion of the FLEX-PCB is shown here. Conventional FLEX-PCB 10 consists of single or multilayer of uniform core layers 12, adhesive used for attaching signal lines and ground to Polyimide 14A and also used for stacking multilayers 14, signal lines 16, and ground 18. The core layer 12 could be any uniform dielectric layer. Usually, Polyimide is used as the core layers in conventional FLEX-PCB. The adhesive 14 is a B-stage (partially cured) acrylic used in between the core layers 12 to stack the multiple core layers. Conventional FLEX-PCB is supplied by vendors as C-staged acrylic (fully cured) adhesive 14A attaching rolled flexible conducting material (usually copper) to the Polyimide core material. The high-speed electrical signal flow through the signal lines 16 attached by adhesive 14 to the core layers 12 and the ground line 18 is laid opposite side of the core layer 12. As shown in FIG. 2, the thickness H, core layer's relative dielectric constant ∈, metal thickness T, and the width W of the signal lines determine the impedance of the signal line. The signal lines 16 can be the microstripline type signal line 16A or stripline type signal line 16B, as shown in FIG. 2. In conventional FLEX-PCB, the microstrip line type signal line 16A in which the ground 18 is separated by the uniform/homogeneous dielectric (core and adhesive) layer 12 and layer 14. Stripline type signal line 16B is also used in conventional FLEX-PCB, in which the signal line 16B is embedded into the homogeneous dielectric (core and adhesive) layer 12 and layer 14, and both sides the ground 18 is used.
Conventional FLEX-PCB 10 as shown in FIG. 1 is manufactured in the way, the flow chart of which is shown in FIG. 3. This is an explanatory diagram for the prior art of FLEX-PCB manufacturing. The dielectric sheet (not shown) 20 is made using the standard FLEX-PCB technology for example using the slurry casting process. The slurry is cast into about 200 μm to 500 μm thick ceramic sheets by slip cast process. The FLEX-PCB core layer 12 is the homogeneous layer usually used in the conventional FLEX-PCB 10. After the patterning and subsequent etching, the signal line is made on side of the core layers. Microvia and subsequent filling process 24 is done, if necessary. Following this, the sheets 26 are laminated together by hot press to form the FLEX-PCB 28. Density heterogeneities in the laminated samples influence any shrinkage in the sintered substrate. Therefore, this lamination process is homogenously carried out by means of the correct dimensional die and punch with flat surfaces. Burn out and sintering process for the multilayered FLEX-PCB board 10, may be necessary after lamination at the temperature suitable to material used as the sheet. The via hole opening and subsequent metal filling (not shown here) are usually done. A sheet 20 may have more than 10,000 via holes in a in a 50 to 500 μm square area.
In conventional FLEX-PCB 10, as the signal line 16 is either laid on the dielectric material (core layer 12) or embedded into the dielectrics, based on the dissipation factor (tangent loss) of the dielectric material used as the core layer in the FLEX-PCB, the signal experiences dissipation while propagating through the signal line 16. The reason is that the electric field starts from signal line and ends in the ground 18 (not shown) and this electric field passes through the dielectric. This signal dispersion is proportional to the signal frequency, i.e., signal speed. It does mean that the higher the signal speed, the lower the distance of transmission of signal for the fixed dielectric material. In the other words, the higher the speed, the lower the bandwidth of the signal line which is used for connecting one chip to other chip on the board. If the tangent loss of the dielectrics are high, the bandwidth of the interconnects gets so limited that, high speed signal can't be sent over longer distance as compared with the dielectrics having the lower tangent loss.
In addition to tangent loss, the dielectric constant of dielectrics material is also important, as electrical field inside dielectric material having higher dielectric constant experiences more signal delay as compared with that of transmission line comprising with lower dielectric constant material. These causes signal skews for the different length signal lines. In this case also, lower dielectric constant material is necessary in the interconnection for high-speed signal interconnection. This is true for both on-chip and off-chip interconnection. Lower dielectric constant material with low dielectric loss offers following functions; Higher density interconnection is possible due to reduction of the cross talk, (2) reducing the capacitance of the interconnection, helping to transfer the signal longer distance, and (3) lower propagation delay.
Considering signal loss and signal delay for various signal line length it is highly desirable to design the interconnects on FLEX-PCB with effective dielectric constant and effective loss of the interconnect system lower.
It is very straight forward that increasing the bandwidth can be possible using of the material having the lower loss tangent (dielectric loss). However, in this case, for off-chip interconnection new material development is necessary. Besides, manufacturing technology is needed to develop to implement in the product level. Conventionally, to increase the interconnects bandwidth, dielectrics having lower tangent loss is used as the FLEX-PCB layer. This dielectric material is very high cost and the manufacturing process for building FLEX-PCB using these materials are not matured yet. In addition, the FLEX-PCB made of such low loss material has low reliability. It is highly desirable to have high speed FLEX-PCB that can be built up with the conventional well-matured dielectric material (for example Polyimide) and also conventional well-matured fabrication process can be used. This can not only reduce the cost, but also have high reliability.
Much work can be found in off-chip interconnection technology focusing on the material development. As for example, low loss materials like Rogers R/flex 1100, etc. are under development stage, to achieve high bandwidth. Implementing new material in FLEX-PCB fabrication process will cost tremendously to make it mature. In addition, new materials having low tangent loss is a material incompatible with conventional dielectric material such as Polyimide processing so is not a low cost solution. These materials will require a much higher temperature and pressure for lamination. Today, in developing the high speed FLEX-PCB, more focused are being paid on shortening the length or on the interconnection layout. In both cases, implementing technology would need to pay high cost.
As explained above, the conventional FLEX-PCB technology being used for off-chip interconnection cannot be used as the need of the signal speed is increasing. And also exiting conventional electrical interconnects have the limitation of achieving the bandwidth in certain level, beyond that complete manufacturing technology is needed to be changed which is costly for FLEX-PCB industries. It is highly desirable to have lower dielectric constant and lower dielectric loss (loss tangent) by adopt a technique or method which can be easily implemented, and which can use the standard dielectric material FLEX-PCB technology.